One common experience I have as an aro person that looking back seems somewhat indicative is the utter lack of crushes and also not understanding crushes. As early as 4th grade I can remember kids teasing each other about having a crush on someone, or someone having a crush on them. I was always that kid who said "no one" when people asked who I had a crush on, and
meant it and I didn't understand why people (my siblings, friends, classmates, etc) insisted it had to be a lie. I would write and read stories involving romance (and there are aro-aces who still want relationships), but I personally couldn't/didn't fantasize about it for
myself. Imagining myself in a romantic relationship to this day is weird, my brain literally cannot compute. Ironically, other people (as early as 14) would also make jokes or comments about how they were so curious to see what I would act in a romantic relationship because it was so removed from my experiences.
So I would say you can always have her experiencing things that end up being common for a lot of aro-ace people, because even as someone who didn't care about romance or really think about it as a child, it still popped up because our society is so romance-obsessed. I'm not sure if you are aro-ace yourself and could use your experiences? But I've seen a few bloggers elsewhere talk about experiences as kids that end up seeming pretty universal for aro-ace people. And having other characters say stuff like "oh you're just a late bloomer" or "you'll understand when you're older" is obviously not a concrete indicator, but it's a pretty common thing to hear so I don't think it would hurt having it in there. I think a lot of people would identify with being told those things.
Also, I don't think it's unreasonable for kids of this generation to be on the Internet even earlier than 14, so like Veinglory said, she could find a discussion about this stuff online (though it's not as common as saying being bi so she might need to be looking up those terms and stumbling upon a discussion?)
Some it will also depend on the type of scenes and other characters you have, so there might be some natural places you can contrast her aro-aceness/lack of caring with other peoples' or general attitudes.
Your question does make sense so I hope that helps! If you want to talk more about it or have more questions, I'm happy to talk more about this